This invention relates to pressure relief systems. More particularly, the present invention relates to an improved rupture disk assembly for a pressure relief system.
Pressure relief assemblies are commonly used as safety devices in systems containing pressurized fluids in gas or liquid form. A pressure relief assembly will vent fluid from the system when the pressure in the system reaches an unsafe level. A number of emergency conditions, including fire and system failure, can create dangerous pressure levels, which require immediate relief to preserve the safety of the system.
Generally, a pressure relief assembly includes a rupture disk that is sealed between a pair of support members, or safety heads. The pressure relief assembly is then typically clamped, or otherwise sealingly disposed, between a pair of conventional pipe flanges in the pressurized system. One of the pipes conducts pressurized fluid to one side of the pressure relief assembly and the other pipe provides an outlet to a safety reservoir or may be open to the environment. The support members include a central opening that exposes a portion of the rupture disk to the pressurized fluid in the system. The exposed portion of the rupture disk will rupture when the pressure of the fluid reaches a predetermined differential pressure between the inlet and outlet sides. The ruptured disk creates a vent path that allows fluid to escape through the outlet to reduce the pressure in the system.
Rupture disks typically have a dome shape and can be either forward acting or reverse acting. In a forward acting disk, the concave side of the dome faces the pressurized fluid, placing the material of the disk under tension. In a reverse acting disk, the convex side of the dome faces the pressurized fluid, placing the material of the disk under compression. In the reverse acting disk (also known as a reverse buckling disk), when the pressure of the fluid exceeds the predetermined level and the material of the disk structure cannot withstand the pressure, the dome of the disk will buckle and begin to reverse. This reversal, or buckling, will begin at a particular point on the disk, known as the point of initial reversal. As the disk continues to reverse, the material of the disk is torn by an opening means to create the vent path to release the pressurized fluid.
Both types of disks commonly include score lines (areas of weakness) to facilitate the opening of the disk. In a reverse buckling disk, the disk will tear along the score line when the disk is reversing. Selected portions of the disk are usually left unscored, acting as a hinge area, to prevent the disk from fragmenting upon bursting and escaping along with the pressurized fluid. Additionally, pressure relief assemblies are known that include safety members to assist in opening the disk and to absorb the energy created by the bursting of the disk to attempt to prevent the disk from fragmenting.
In an emergency situation, where the system pressure becomes unsafe, it is important to reduce the pressure as quickly as possible. The American Society of Mechanical Engineers (ASME) code establishes minimum performance requirements for fluid flow rates through pressure relief systems. The size and shape of the opening created when the disk bursts is a limiting factor on the rate at which fluid can escape the system. A burst disk having a large, unobstructed opening will perform better than a burst disk having a small, obstructed opening because the velocity head loss (i.e. pressure drop) over the large, unobstructed opening will be lower than the velocity head loss over a smaller or obstructed opening. The lower velocity head loss translates to a lower flow resistance (K.sub.r) and, thus, a greater flow rate through the disk.
Adjusting different facets of the disk design, including the size of the rupturable portion of the disk and the location of the score line, can control the size and shape of the opening created when the disk bursts. A larger disk has the potential to create a larger opening.
Another factor affecting flow resistance is the nature of the fluid in the pressurized system. It has been found that rupture disks open differently depending on the nature of the fluid in the system. Typically, a disk burst in a gas environment will open more fully than a disk burst in a liquid environment. Thus, to meet desirable flow resistance performance requirements, the design of a disk may have to be different if the disk is being used in a liquid application, even if the liquid is at the same pressure as a similar gas application.
An additional factor of disk design that affects flow resistance is the thickness of the rupturable portion of the disk. A disk made of a thinner material will bend easier than a disk made of a thicker material. Thus, for disks rupturing at the same fluid pressure, a thinner disk is more likely to completely open and create a large, unobstructed opening than a corresponding thicker disk.
However, a disk made of a thinner material is more susceptible to damage than a thicker disk. Any damage to the rupture disk could alter the actual burst pressure of the disk. This is particularly an issue in low pressure, reverse buckling disks where the disk material must be thin to burst at the desired low pressure. If the disk is damaged during installation, the structural integrity of the disk may be compromised causing the disk to reverse at a pressure significantly less than the desired rupture pressure. In these situations, the material of the disk does not tear as expected and the disk may completely reverse without tearing. The reverse buckling disk then acts like a forward acting disk and the fluid pressure places the material of the disk in tension. Because the tensile strength of the disk material is greater than the corresponding compressive strength, the disk may not tear to create the vent path until the pressure of the system significantly exceeds the desired rupture pressure. This over-pressure condition could result in damage to the system that the rupture disk was intended to prevent.
Rupture disks are rated by their performance in a damaged condition. This rating is generally known as the damage safety ratio of the disk and is determined by dividing the actual pressure at which a damaged disk ruptures by the desired, or rated, rupture pressure of the disk. A damaged disk with a damage safety ratio of 1 or less will burst at the desired rupture pressure, or before the pressurized fluid reaches the desired pressure, thereby preventing any damage to the system.
Another important performance rating of a rupture disk is the burst accuracy of the disk. There are variations in materials, manufacturing, and installation that may result in any given two disks in a manufacturing lot of seemingly identical disks not bursting at the same pressures. Thus, there is typically a variation in actual burst pressure among disks having the same rated pressure. With current rupture disk design and manufacturing methods, rupture disks will typically burst at a pressure that is .+-.5% of the rated pressure or .+-.2 psig when the rated pressure is below 40 psig. Thus, to prevent premature disk rupture and to provide a safety margin, the standard operating pressure of a system should not exceed 90% of the rated pressure of a rupture disk used in the system.
In light of the foregoing, there is a need for a pressure relief assembly that provides a low flow resistance K.sub.r in both liquid and gas applications. There is further a need for rupture disks that have an accurate and repeatable burst pressure and thus can be used in a high operating capacity. There is still further a need for a rupture disk having a low damage safety ratio so that an inadvertently damaged reverse buckling disk does not create a potentially dangerous over-pressure situation in either liquid or gas applications.